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[RFC PATCH v4 4/8] x86/apic: Introduce Remote Action Request Operations

Rik van Riel posted 8 patches 3 months, 3 weeks ago
[RFC PATCH v4 4/8] x86/apic: Introduce Remote Action Request Operations
Posted by Rik van Riel 3 months, 3 weeks ago 
From: Yu-cheng Yu <yu-cheng.yu@intel.com>

RAR TLB flushing is started by sending a command to the APIC.
This patch adds Remote Action Request commands.

Because RAR_VECTOR is hardcoded at 0xe0, POSTED_MSI_NOTIFICATION_VECTOR
has to be lowered to 0xdf, reducing the number of available vectors by
13.

[riel: refactor after 6 years of changes, lower POSTED_MSI_NOTIFICATION_VECTOR]

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Signed-off-by: Rik van Riel <riel@surriel.com>
---
 arch/x86/include/asm/apicdef.h     | 1 +
 arch/x86/include/asm/irq_vectors.h | 7 ++++++-
 arch/x86/include/asm/smp.h         | 1 +
 arch/x86/kernel/apic/ipi.c         | 5 +++++
 arch/x86/kernel/apic/local.h       | 3 +++
 5 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/apicdef.h b/arch/x86/include/asm/apicdef.h
index 094106b6a538..b152d45af91a 100644
--- a/arch/x86/include/asm/apicdef.h
+++ b/arch/x86/include/asm/apicdef.h
@@ -92,6 +92,7 @@
 #define		APIC_DM_LOWEST		0x00100
 #define		APIC_DM_SMI		0x00200
 #define		APIC_DM_REMRD		0x00300
+#define		APIC_DM_RAR		0x00300
 #define		APIC_DM_NMI		0x00400
 #define		APIC_DM_INIT		0x00500
 #define		APIC_DM_STARTUP		0x00600
diff --git a/arch/x86/include/asm/irq_vectors.h b/arch/x86/include/asm/irq_vectors.h
index 47051871b436..52a0cf56562a 100644
--- a/arch/x86/include/asm/irq_vectors.h
+++ b/arch/x86/include/asm/irq_vectors.h
@@ -97,11 +97,16 @@
 
 #define LOCAL_TIMER_VECTOR		0xec
 
+/*
+ * RAR (remote action request) TLB flush
+ */
+#define RAR_VECTOR			0xe0
+
 /*
  * Posted interrupt notification vector for all device MSIs delivered to
  * the host kernel.
  */
-#define POSTED_MSI_NOTIFICATION_VECTOR	0xeb
+#define POSTED_MSI_NOTIFICATION_VECTOR	0xdf
 
 #define NR_VECTORS			 256
 
diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index 0c1c68039d6f..0e5ad0dc987a 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -120,6 +120,7 @@ void __noreturn mwait_play_dead(unsigned int eax_hint);
 void native_smp_send_reschedule(int cpu);
 void native_send_call_func_ipi(const struct cpumask *mask);
 void native_send_call_func_single_ipi(int cpu);
+void native_send_rar_ipi(const struct cpumask *mask);
 
 asmlinkage __visible void smp_reboot_interrupt(void);
 __visible void smp_reschedule_interrupt(struct pt_regs *regs);
diff --git a/arch/x86/kernel/apic/ipi.c b/arch/x86/kernel/apic/ipi.c
index 98a57cb4aa86..9983c42619ef 100644
--- a/arch/x86/kernel/apic/ipi.c
+++ b/arch/x86/kernel/apic/ipi.c
@@ -106,6 +106,11 @@ void apic_send_nmi_to_offline_cpu(unsigned int cpu)
 		return;
 	apic->send_IPI(cpu, NMI_VECTOR);
 }
+
+void native_send_rar_ipi(const struct cpumask *mask)
+{
+	__apic_send_IPI_mask(mask, RAR_VECTOR);
+}
 #endif /* CONFIG_SMP */
 
 static inline int __prepare_ICR2(unsigned int mask)
diff --git a/arch/x86/kernel/apic/local.h b/arch/x86/kernel/apic/local.h
index bdcf609eb283..833669174267 100644
--- a/arch/x86/kernel/apic/local.h
+++ b/arch/x86/kernel/apic/local.h
@@ -38,6 +38,9 @@ static inline unsigned int __prepare_ICR(unsigned int shortcut, int vector,
 	case NMI_VECTOR:
 		icr |= APIC_DM_NMI;
 		break;
+	case RAR_VECTOR:
+		icr |= APIC_DM_RAR;
+		break;
 	}
 	return icr;
 }
-- 
2.49.0
Re: [RFC PATCH v4 4/8] x86/apic: Introduce Remote Action Request Operations
Posted by Kirill A. Shutemov 3 months, 2 weeks ago 
On Thu, Jun 19, 2025 at 04:03:56PM -0400, Rik van Riel wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
> 
> RAR TLB flushing is started by sending a command to the APIC.
> This patch adds Remote Action Request commands.
> 
> Because RAR_VECTOR is hardcoded at 0xe0, POSTED_MSI_NOTIFICATION_VECTOR
> has to be lowered to 0xdf, reducing the number of available vectors by
> 13.
> 
> [riel: refactor after 6 years of changes, lower POSTED_MSI_NOTIFICATION_VECTOR]

But why? Because it is used as FIRST_SYSTEM_VECTOR?

> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Signed-off-by: Rik van Riel <riel@surriel.com>
> ---
>  arch/x86/include/asm/apicdef.h     | 1 +
>  arch/x86/include/asm/irq_vectors.h | 7 ++++++-
>  arch/x86/include/asm/smp.h         | 1 +
>  arch/x86/kernel/apic/ipi.c         | 5 +++++
>  arch/x86/kernel/apic/local.h       | 3 +++
>  5 files changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/apicdef.h b/arch/x86/include/asm/apicdef.h
> index 094106b6a538..b152d45af91a 100644
> --- a/arch/x86/include/asm/apicdef.h
> +++ b/arch/x86/include/asm/apicdef.h
> @@ -92,6 +92,7 @@
>  #define		APIC_DM_LOWEST		0x00100
>  #define		APIC_DM_SMI		0x00200
>  #define		APIC_DM_REMRD		0x00300
> +#define		APIC_DM_RAR		0x00300

Hm. Do we conflict with APIC_DM_REMRD here?

>  #define		APIC_DM_NMI		0x00400
>  #define		APIC_DM_INIT		0x00500
>  #define		APIC_DM_STARTUP		0x00600

-- 
  Kiryl Shutsemau / Kirill A. Shutemov
Re: [RFC PATCH v4 4/8] x86/apic: Introduce Remote Action Request Operations
Posted by Sean Christopherson 3 months, 2 weeks ago 
On Thu, Jun 26, 2025, Kirill A. Shutemov wrote:
> On Thu, Jun 19, 2025 at 04:03:56PM -0400, Rik van Riel wrote:
> > From: Yu-cheng Yu <yu-cheng.yu@intel.com>
> > 
> > RAR TLB flushing is started by sending a command to the APIC.
> > This patch adds Remote Action Request commands.
> > 
> > Because RAR_VECTOR is hardcoded at 0xe0, POSTED_MSI_NOTIFICATION_VECTOR
> > has to be lowered to 0xdf, reducing the number of available vectors by
> > 13.
> > 
> > [riel: refactor after 6 years of changes, lower POSTED_MSI_NOTIFICATION_VECTOR]
> 
> But why? Because it is used as FIRST_SYSTEM_VECTOR?

The Posted MSI Notifications vector should be the lowest of the system vectors
so that device IRQs are NOT prioritized over "real" system vectors.

> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> > Signed-off-by: Rik van Riel <riel@surriel.com>
> > ---
> >  arch/x86/include/asm/apicdef.h     | 1 +
> >  arch/x86/include/asm/irq_vectors.h | 7 ++++++-
> >  arch/x86/include/asm/smp.h         | 1 +
> >  arch/x86/kernel/apic/ipi.c         | 5 +++++
> >  arch/x86/kernel/apic/local.h       | 3 +++
> >  5 files changed, 16 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/include/asm/apicdef.h b/arch/x86/include/asm/apicdef.h
> > index 094106b6a538..b152d45af91a 100644
> > --- a/arch/x86/include/asm/apicdef.h
> > +++ b/arch/x86/include/asm/apicdef.h
> > @@ -92,6 +92,7 @@
> >  #define		APIC_DM_LOWEST		0x00100
> >  #define		APIC_DM_SMI		0x00200
> >  #define		APIC_DM_REMRD		0x00300
> > +#define		APIC_DM_RAR		0x00300
> 
> Hm. Do we conflict with APIC_DM_REMRD here?

Yes and no.  Yes, it literally conflicts, but it's easy enough to define the behavior
of APIC_DM_{REMRD,RAR} based on feature support.  E.g. KVM is likely going to add
support for Remote Read, which would conflict with KVM's bastardization of
APIC_DM_REMRD for PV kicks.  But as Paolo pointed out[*], KVM's PV unhalt/kick
can simply be gated on KVM_FEATURE_PV_UNHALT.  Any code that cares should be able
to do the same thing for RAR.  E.g. KVM's code could end up being something like:

	case APIC_DM_REMRD:
		if (guest_pv_has(vcpu, KVM_FEATURE_PV_UNHALT)) {
			result = 1;
			vcpu->arch.pv.pv_unhalted = 1;
			kvm_make_request(KVM_REQ_EVENT, vcpu);
			kvm_vcpu_kick(vcpu);
		} else if (guest_has_rar(vcpu)) {
			<magic!>
		} else {
			<emulate legacy Remote Read>;
		}
		break;

For the kernel itself, there's nothing to do, because Linux doesn't use Remote Read.

[*] https://lore.kernel.org/all/CABgObfadZZ5sXYB0xR5OcLDw_eVUmXTOTFSWkVpkgiCJmNnFRQ@mail.gmail.com

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